A refrigeration and air-conditioning apparatus formed of a compressor, an outdoor heat exchanger, an indoor-side expansion device, and an indoor heat exchanger connected to each other via a refrigerant circuit has been conventionally used. In some cases, such a refrigeration and air-conditioning apparatus is provided with an oil separator to separate refrigerant and a refrigerating machine oil mixed into refrigerant and brought out of the compressor, and an oil return circuit to return oil to the compressor. The oil return circuit is a pipe for connecting the oil separator on a discharge side of the compressor to a suction side of the compressor. The refrigerating machine oil brought out of the compressor is returned to the suction side of the compressor by the oil return circuit to prevent the refrigerating machine oil from flowing into an indoor-unit side pipe, and thereby oil level lowering in the compressor is prevented. In the oil return circuit, a predetermined constant amount of oil is returned, and a flow rate cannot be transitionally adjusted in some cases.
In contrast, in Patent Literature 1, a technique providing a solenoid valve that opens and closes in response to an oil level of a refrigerating machine oil accumulated in a compressor is suggested. Moreover, in Patent Literature 2, a technique for opening and closing a solenoid valve in response to a refrigerant concentration in a compressor is suggested. In both literatures, when an oil level in the compressor is not more than a certain value, the oil can be emergently returned.